The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission control ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmissio ratio control valve operates to determine the transmission ratio in accordance with a control signal dependent on the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance a control signal dependent on with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission rato is automatically and continuously reduced at a rate which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the rate of changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which decreases the driveability of the vehicle.
European patent application No. 205257 discloses a transmission ratio control system in which a transmission ratio changing rate is di/dt is obtained by making calculation K(id-i), where id is desired transmission ratio and i is actual transmission ratio. In order to make the calculation, memories and calculator sections are provided. Accordingly, the system and program become complicated.